If Gov. Pat Quinn signs legislation backed by Mayor Rahm Emanuel to alter the Chicago Park District pension plan, the measure could head straight to court, where it would become a test case on Illinois constitutional protections of pension benefits, union officials said Friday.

"We believe it's unconstitutional," said Adam Rosen, spokesman for Service Employees International Union Local 73, which represents more than 80 percent of the district's 3,000 or so year-round employees. "We are already looking for members to be representatives in a lawsuit. ... Hopefully it does not have to get that far."

Rosen said his union will lobby Quinn not to sign the bill. That would leave Quinn, who faces re-election next year, with a political dilemma: sign the bill and alienate unions, or veto it and lose a chance to claim some small measure of pension reform success. The governor's office is reviewing the bill, spokeswoman Brooke Anderson said.

Sarah Hamilton, the mayor's spokeswoman, said the proposal was developed with the unions and would pass constitutional muster. "What we all agree on is that the pension fund needs to be funded to ensure the retirement security of the employees, so both workers and taxpayers have certainty," she said.

The bill was approved this week by both the state House and Senate, with legislators conceding all the unions were not on board in the end. The legislation would increase both employee and district contributions to the pension plan, increase some retirement ages and lower cost-of-living adjustments.

SEIU Local 73 had agreed to the increased contributions and reduced COLAs for current employees but not reduced COLAs for already retired workers, Rosen said.

Emanuel lauded the plan for its "balanced approach" to bringing an underfunded pension system into financial alignment and suggested that same type of approach could serve as a template for the city's four pension funds, which are underfunded by $19.5 billion.

But that seems unlikely, with a coalition of unions on Friday saying the approach "unfairly slashes constitutionally protected pension benefits" — a reference to a clause in the Illinois Constitution that pension benefits shall not be diminished or impaired.